Alarm system with air pressure detector

ABSTRACT

An intrusion alarm system in which intrusion into an protected space is detected as a variation in air pressure. The variable pressure detector uses a membrane and a displacement detector. One side of the membrane is exposed to the protected space and the opposite side of the membrane is enveloped by an enclosure with a limited pressure coupling to the protected space. A signal from the displacement detector is analyzed by a processor to identify rapid changes in air pressure to activate the security alarm. The same type of a variable pressure detector may be used to control electric lights and other devices in response to people entering into a room.

The invention relates to alarm systems, and in particular to an alarmsystem designed to protect an enclosed space and give warning that thespace has been penetrated by an intruder. It is based on U.S.Provisional Patent Application No. 60/842,522 filed on Sep. 6, 2006.

BACKGROUND OF THE INVENTION

An intrusion alarm is typically intended to protect an enclosed spacefrom intrusion. The space may be a domestic dwelling or commercialbuilding, a room in such a building, a safe, a vault, or the interior ofa vehicle.

It is a well known fact that air pressure in an enclosed space willremain unchanged as long as that space remains fully enclosed. When thespace develops an opening, air pressure changes depending on the outsideair pressure. If the enclosed space is a room in a building, airpressure inside will remain either constant or will change slowly inaccordance with the outside atmospheric pressure. Opening of doors andwindows would result in a rapid fluctuation of the air pressure in theroom. This can be detected by an appropriate sensor.

In U.S. Pat. No. 3,947,838 there is described an alarm system comprisinga moving vane sensor responsive to air pressure within an enclosedspace, the sensor providing electrical signals related to the sensed airpressure, and a signal processor to which the electrical signals aresupplied and operative to initiate an alarm indication when the signalsupplied by the sensor is indicative of an intrusion into the enclosedspace.

The U.S. Pat. No. 4,692,734 issued to Holden et al. describes the signalprocessing in the alarm system based on a comparison of the currentsignal with the reference set.

The prior art relies on use of either complex pressure sensors, or thepressure sensors are not sufficiently sensitive to detect as smallpressure variations as few mm H₂O.

It is therefore the object of this invention to develop a sensor for thesecurity alarm system that is sensitive to detect small changes inpressure;

It is another object of this invention to make pressure sensorinsensitive to slow changing air pressure.

And another object of this invention is to reduce a complexity and costthe air pressure sensor.

SUMMARY OF THE INVENTION

According to this invention an alarm system comprises a sensorresponsive to air pressure changes within an enclosed space. The sensorcontains a thin and relative large membrane with one side exposed to theair in a monitored enclosed space, while the opposite side of themembrane is enveloped by an enclosure having a small hole that isexposed to the same monitored enclosed space. The hole restricts the airflow between the interior of the enclosure and the outside, thusresulting in a delay between the variations in pressure inside andoutside of the enclosure. The delay causes a temporary disbalance ofpressures across the membrane and thus the membrane deflection. Thedeflection is measured by the displacement sensor, for example, optical.The output signal of the displacement sensor is further compared with apredetermined threshold whose output, in turn, controls the alarm.

DESCRIPTION OF THE DRAWINGS

This invention will now be described by way of example with reference tothe drawings, in which:

FIG. 1 is an example of an enclosed space with a door and windows;

FIG. 2 shows variations in air pressure within the enclosed space;

FIG. 3 is a cross-sectional view and block-diagram of the differentialair pressure sensor and the alarm system;

FIG. 4. shows operation of the optical displacement detector;

FIG. 5 depicts a timing diagram of pressures across the membrane, and

FIG. 6 shows an opening in the enclosure with a variable aperture.

FIG. 7 illustrates a capacitive option of the displacement sensing, and

FIG. 8 depicts a corrugated membrane.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The system shown in FIG. 1 comprises a sensor 5 which is arranged in anenclosed space 1 to be monitored and responsive to the air pressure inthat space to provide electrical signals indicative of the air pressurevariations at any time. The sensor 5 is connected to the monitor 6 thatmay be comprised of a microprocessor, alarm, power source and othercomponents. The enclosed space 1 has windows 3 and one or more doors 4.The interior air pressure is P_(h) and the exterior pressure isatmospheric P_(atm). Generally, these pressures are somewhat different,primarily due to a temperature gradient between the enclosed space andthe outside. When the doors and windows are closed, still some air leaksmay be present and pressure P_(h) would change rather slowly along withP_(aim). When doors 4 or windows 3 are opened and closed, air flow(draft) occurs and pressure P_(h) changes more rapidly towardsequalization with P_(aim). The same effect occurs when people enter theenclosed space and move within the space. This is illustrated in FIG. 2that shows the internal air pressure. When it changes slowly, thechanges Δ_(a) are smaller than Δ_(b) which occur during the rapidpressure variations. The time to is a fixed interval to measure thepressure variations. The purpose of sensor 5 is to respond to fasterchanges in pressure and not to respond to slower changes in pressure. Italso should be noted that air drafts caused by movement of intruders maybe quite small—typically not greater than few mmH₂O.

A differential pressure sensor 5 is shown in FIG. 3. Unlike theconventional differential pressure sensors that respond to constant andchanging pressures, the illustrated sensor responds only to relativelyfast changes in the gas pressure differential and is not sensitive toslow changing pressures. A goal of the sensor is to convert thedifferential air (gas) pressure changes to the output electrical signalthat can be processed by the signal conditioner 20, processor 22 andactivate the alarm 23, if needed. In this example of the design, theprinted circuit board (PCB) 10 supports membrane 13 which is air-tightsealed to the PCB 10 all around the circumference at areas 14 and 15.The PCB acts as a support structure. The membrane is fabricated of anysuitable material, such as Mylar, aluminum or brass foil and isstretched reasonably tight. It must be flexible enough to respond tosmall variations in pressure across its thickness. A shape of themembrane 13 may be a disk having a diameter from 0.25 to 4 inch andthickness between 0.0005 and 0.005 of an inch. The membrane may be flator corrugated as shown in FIG. 8 where the creases 46 may have acircular shape.

Next to the membrane 13, the PCB 10 has an opening 11 which is smallerthan the membrane overall size. An inlet tube 12 is attached to the PCB10 to allow air pressure P_(h) to access the membrane 13 through theopening 11. At the opposite side of the PCB 10, there is an enclosure 16which is air-tightly attached to the PCB 10. The membrane 13 has twosides: side 50 is exposed to the protected space, while side 51 isexposed to enclosure 16. In other words, membrane 13 at the left side 50is exposed to the monitored pace air pressure P_(h), while at the rightside 51 it is exposed to the air pressure P₂ inside the enclosure 16.The enclosure 16 has at least one hole 17 whose aperture may be eitherfixed or adjusted by a moving cover 34 as illustrated in FIG. 6. Thecover 34 may be rotated around pivot 35. In general, the area ofaperture of the hole 17 shall be at least 100 times smaller than theoverall inner surface area of the enclosure 16 or the membrane 13.

In the first preferred embodiment, at one of the sides of the membrane13, for example at side 51, there is a displacement sensor 18 asillustrated in FIG. 3. The purpose of the displacement sensor 18 is todetect the membrane 13 displacement, that is, to convert distance 19 tothe membrane 13 into electrical signal that can be processed by thesignal conditioner 20. The membrane 13 displacement is the measure of adifferential pressure ΔP.

Since the enclosure 16 is connected to the protected space only througha small hole 17, changes in air pressure P_(h) are not immediatelyreflected by the internal pressure P₂. In other words, there is a phaseshift between the outside and the inside pressures, as illustrated inFIG. 5. When P_(h) changes slowly, a small hole 17 allows P₂ to followP_(h) very closely so pressures at both sides of membrane 13 are nearlythe same and the membrane is substantially flat and not moving. Duringfaster changes in P_(h), the hole 17 slows down the pressureequalization and the internal pressure P₂ (dotted line in FIG. 5) lagsbehind and also is somewhat smoother. A differential pressure ΔP acrossthe membrane 13 is shown at the bottom portion of FIG. 5 as pressure 32.When the differential pressure 32 is near zero, the membrane remainssubstantially flat and the distance 19 is at its base level. Whenpressure 32 deflects from zero, the membrane 13 flexes inwardly oroutwardly, thus modulating distance 19.

The displacement sensor 18 monitors this distance 19 and provides asignal to the signal conditioner. When the pressure differential ΔP and,subsequently, the distance 19 are sufficiently large to reach the presetthreshold 33, the processor 22 detects the threshold crossing 36 andindicates the alarming event.

There are numerous ways of designing a displacement sensor. FIG. 4illustrates one possible way of designing the displacement sensor 18. Itis comprised of an opto-coupler 27 with the photo emitter 28 and photodetector 29. The membrane 13 is shown in two states: the base state 25which corresponds to a zero differential pressure, and a flexed state 26when P_(h) is higher than P₂. The right side of membrane 13 is madereflective. For example, if the membrane is made of a plastic film, likeMylar, at least one side can be metallized. When the membrane 13 is instate 25, the emitted light L_(e) is reflected from the membrane andgoes to the detector 29 as the beam L_(r0). The output signal from theopto-coupler 27 is the strongest. When the membrane 13 moves to thestate 26, the reflected light beam L_(p) is diverted from the detector29, causing the opto-coupler's output signal to drop. To minimize theopto-coupler power consumption, the emitted light doesn't need to becontinuous, it can be emitted as short pulses with a small duty cycle.For example, a light pulse can have a duration of 10 microseconds andthe pulses are emitted with a rate of 100 pulses per second. Thiscorresponds to a duty cycle of 0.001 which results in a significantreduction in power consumption without compromising reliability of theintrusion detection.

In the second embodiment, the function of a displacement sensor may beassumed by the signal conditioner 20 that should be responsive tochanges in a capacitance. In this case, the enclosure 16 is replaced bya substantially flat and rigid plate 40 shown in FIG. 7. The disk has atleast one and possibly several small holes 41 whose combined area ofaperture shall be at least 100 times smaller than area of the plate 40adjacent to the membrane. The plate 40 is positioned close to membrane13 and is separated from it by a spacer 43, 44. The gap 42 between themembrane 13 and plate 40 should be no larger than 0.1 of an inch. Theplate 40 shall be electrically conductive and at least one side ofmembrane 13 also shall be electrically conductive. An electricalcapacitance is formed between the membrane 13 and plate 40. A value ofthis capacitance will change when pressure P_(h) varies with respect tothe air pressure P₂ inside the gap 42. The capacitance variations aremeasured by the signal conditioner 20 and presented as the output 45reflecting the differential pressure ΔP.

One should not overlook other potential applications of the abovedescribed differential pressure detector. These may include turning onelectric lights in a room in response to an intrusion or walking nearthe detector. This can be exemplified by a stairway that needs to beilluminated. Traditional infrared motion detectors that are used forthis purpose respond only when there is a direct vision of the intruder,while the differential air pressure detector would have a coverage notlimited by a direct line of view. In such applications, an alarm 23 ofFIG. 3 is replaced with an electric switch.

Without further elaboration, the foregoing will so fully illustrate ourinvention that others may, by applying current or future knowledge,readily adopt the same for use under various conditions of service.

1. A detector of a variable gas pressure in a monitored space comprisingthe membrane having first side and second side, wherein the first sideis exposed to a monitored space filled with gas, such membrane is beingattached to a support structure; the enclosure adjacent to the secondside of said membrane to envelop a volume of gas near the second side ofthe membrane; a hole being formed in said enclosure to pneumaticallyconnect the enveloped volume of gas within said enclosure to themonitored space.
 2. A detector of a variable gas pressure of claim 1where said hole has a cross-sectional area at least 100 times smallerthan the inner surface area of said enclosure.
 3. A detector of avariable gas pressure of claim 1 where said membrane has at least oneside being composed of metal.
 4. A detector of a variable gas pressureof claim 1 further comprising a displacement sensor being responsive tomovement of said membrane.
 5. A detector of a variable gas pressure ofclaim 1 further comprising a support structure being attached to saidenclosure;
 6. A detector of a variable gas pressure of claim 1 wheresaid hole has an adjustable aperture.
 7. The intrusion alarm comprisinga detector of a variable air pressure, a signal processing circuit andan alarm, wherein said detector of a variable air pressure is moreresponsive to faster changes in the air pressure and less responsive toslower changes in the air pressure.
 8. The intrusion alarm of claim 7where said signal processing circuit further comprises a thresholddetector responsive to a difference between the faster changes in theair pressure and slower changes in the air pressure.
 9. An electricswitch that closes the electric circuit in response to variations airpressure in a monitored space, comprising in combination a differentialair pressure detector being comprised of a membrane having two sides,where one side is directly exposed to air in the monitored space, whilethe other side is exposed to air in the monitored space through a holebeing smaller than the membrane; a signal conditioning circuit forprocessing signals from said differential air pressure detector; athreshold detector; an electric switch being controlled by saidthreshold detector